Their battle to succeed veteran Sen. Tom Harkin, a Democrat retiring after 30 years, is one of the most closely watched in the nation.

“If you drive north into Iowa on I-35 coming from Missouri, you’ll see a landscape full of red counties far to the left and blue counties far to the right,” David Paleologos, director of the Suffolk University Political Research Center in Boston, said in a statement.

“The poll pits Democrat Braley’s congressional district in the northeast against Ernst’s Senate district in the southwest, but the voters in between will make the difference,” he said.

Independents could make the difference. Braley leads 80-5 among Democrats. Ernst is up 77-5 among Republicans. Independents prefer Ernst, 39-36 percent.